The present invention relates to a process for the preparation of copolymers of styrene or .alpha.-methylstyrene with an unsaturated acid derivative, and optionally, additional comonomers.
It has recently been required that the fixing temperature of toners for electrophotographic devices be lowered to reduce the power consumption of these devices. One convenient method for lowering the fixing temperature is to reduce the molecular weight of the toner. However, the use of a low molecular weight compound alone as the toner material has the disadvantage that it causes blocking. Further, the use of a high molecular weight resin is essential for imparting offset properties to a toner. Thus, a resin for a toner must consist of both a low molecular weight part and a high molecular weight part in order to provide fixability at low temperatures, offset properties and anti-blocking properties.
In attempts to obtain an acceptable polymeric material, a polymerization process characterized by the use of a reduced amount of polymerization initiator and a lower polymerization temperature was suggested. However, such process has the disadvantage of taking an uneconomically long time. The use of a difunctional peroxide as the initiator in this process was also proposed, but the process still did not produce satisfactory results. Thus, there is a need for a polymerization process which produces polymeric products suitable for today's needs in the electrophotographic reproduction field.
In another field in which the products of the present invention are useful, it has been required to develop copolymers of a .alpha.-methylstyrene having a high thermal deformation temperature, high heat stability, excellent transparency and which are produced with a high degree of polymerization. In this area, copolymerization processes have been employed using t-butylperoxy benzoate, t-butylperoxy acetate and benzoyl peroxide as the polymerization initiator. However, these polymerization initiators are required in large amounts and thus, often cause significant discoloration of the copolymer and providing an unacceptably low degree of polymerization.
Emulsion polymerization was also employed to produce a copolymer with .alpha.-methylstyrene having a high degree of polymerization and a high conversion. However, the copolymers prepared by emulsion polymerization have the unavoidable disadvantage that they become contaminated with an emulsifier during the polymerization process resulting in discoloration of the polymer. Thus, there is a need for a polymerization process for producing a .alpha.-methylstyrene-containing copolymers which are transparent, have a high degree of polymerization and exhibit excellent thermal resistance.
It has now been found that styrene and .alpha.-methylstyrene-containing copolymers suitable for the above-mentioned uses may be prepared by a copolymerization process wherein styrene or .alpha.-methylstyrene is copolymerized with an unsaturated acid derivative in the presence of any certain free radical initiators.
The peroxide cross-linking compound used in the present process is disclosed in U.S. Pat. No. 3,342,872 as being suitable for cross-linking of natural and synthetic rubbers to substantially reduce the disagreeable odor associated with such cross-linked rubbers. In addition, U.S. Pat. No. 3,357,964 discloses the peroxide compound used in the present process for use in the polymerization of vinylaromatic monomers such as styrene, .alpha.-methylstyrene, vinyltoluene, vinylcarbazole and mixtures of these compounds. It is said that this polymerization process is faster than previous processes and allows one to obtain higher molecular weight polymers. However, these patents do not disclose the use of this peroxide as an initiator for a process such as that of the present invention wherein copolymers of styrene or .alpha.-methylstyrene with unsaturated acid derivatives are produced. Further, these patents do not teach or suggest that the significant, unexpected properties of the products of the present process would be obtained by such a process.
A process for the preparation of copolymers of styrene and acrylic acid is disclosed in European Patent Application 0 002 035. In that process, styrene-acrylic acid copolymers are prepared by the mass copolymerization of styrene and acrylic acid in the presence of difunctional peroxide initiators.
Additionally, a process for the preparation of .alpha.-methylstyrene/acrylonitrile copolymers by suspension or bulk polymerization is disclosed in European Patent Application 0 107 795. The process is carried out in the presence of a difunctional peroxide initiator.